HADB Performance Problems

Performance is affected when the transactions to HADB get delayed or aborted. This situation is generally caused by a shortage of system resources. Any wait beyond five seconds causes the transactions to abort. Any node failures also cause the active transaction on that node at crash time to abort. Any double failures (failure of both mirrors) will make the HADB unavailable. The causes of the failures can generally be found in the HADB history files.

Is There a Shortage of CPU or Memory Resources, or Too Much Swapping?

Description

Node restarts or double failures due to “Process blocked forxsec, max block time is 2.500000 sec.” In this case, x is the length of time the process was blocked, and it was greater than 2.5 seconds.

The HADB Node Supervisor Process (NSUP/clu_nsup_srv) tracks the time elapsed since the last time it did some monitoring work. If that time duration exceeds a specified maximum (2500ms by default), NSUP concludes that it was blocked too long and restarts the node.

NSUP being blocked for more than 2.5 seconds cause the node to restart. If mirror nodes are placed on the same host, the likelihood of double failure is high. Simultaneous occurrence of the blocking on the mirror hosts may also lead to double failures.

The situation is especially likely to arise when there are other processes—for example, in a colocated configuration— in the system that compete for CPU, or memory which produces extensive swapping and multiple page faults as processes are rescheduled.

NSUP being blocked can also be caused by negative system clock adjustments.

Solution

Ensure that HADB nodes get enough system resources. Ensure also that the time synchronization daemon does not make large (not higher than 2 seconds) jumps.

Is There Disk Contention?

Description

A disk contention can have a negative impact on user data read/writes to the disk devices, as well as on HADB writing to history files. Severe disk contention may delay or abort user transactions. Delay in history file writing may cause node restarts and, in the worst case, lead to double failures.

The disk contention can be identified by monitoring the disk I/O from the OS, for the disks used for data, log devices and history files. History file write delays are written to the HADB history files. This can be identified by “NSUP BEWAREtimestampLast flush took too long (xmsecs).”

This warning shows that disk I/O took too long. If the delay exceeds ten seconds, the node supervisor restarts the trans process with the error message:

This message indicates that a trans (clu_trans_srv) process has been too busy doing other things (for example, waiting to write to the history file) to reply to the node supervisor’s request checking the heartbeat of the trans process. This causes the nsup to believe that the trans has died and then restarts it.

When the operating system is overloaded with too many processes (many HADB nodes co-located with other processes), the scheduling of I/O operations may be delayed. When the HADB related I/O work is delayed, HADB nodes write the following in the history files, “HADB warning: Schedule of async <read,write\> operation took ...”

This problem is observed especially in Red Hat AS 2.1 when multiple HADB nodes are placed on the same host and all the nodes use the same disk to place their devices.

Solution

Use one disk per node to place the devices used by that node. If the node has more than one data devices and the disk contention is observed, move one data device to another disk. The same applies to the history file as well.

Make sure that all data and log devices and all history files reside on local disks (not NFS-mounted or other remotely mounted disks).

If the monitoring tools still show contention on the HADB disks, the data buffer pool size can be increased.

Is There a Shortage of HADB Data Device Space?

Description

One possible reason for transaction failure is running out of data device space. If this situation occurs, HADB will write warnings to the history file, and abort the transaction which tried to insert and/or update data.

Typical messages are:

HIGH LOAD: about to run out of device space, ...
HIGH LOAD: about to run out of device space on mirror node, ...

The general rule of thumb is that the data devices must have room for at least four times the volume of the user data. Please refer to the Tuning Guide for additional explanation.

Solution 1

Increase the size of the data devices using the following command:

hadbm set DeviceSize=size

This solution requires that there is space available on the physical disks which are used for the HADB data devices on all nodes.

HADBM automatically restarts each node of the database.

Solution 2

Stop and delete the HADB, and create a new instance with more nodes and/or larger data devices and/or several data devices per node. Unfortunately, using this solution will erase all persistent data and the schemas created by the Application Server. See the Administrator's Guide for more information about this procedure.

Is There a Shortage of Other HADB Resources?

When an HADB node is started, it will allocate:

Several shared memory segments of fixed size

Internal data structures of fixed size

If an HADB node runs out of resources it will delay and/or abort transactions. Resource usage information is shipped between mirror nodes, so that a node can delay or abort an operation which is likely to fail on its mirror node.

Transactions that are delayed repeatedly may time out and return an error message to the client. If they do not time out, the situation will be visible to the client only as decreased performance during the periods in which the system is short on resources.

These problems frequently occur in “High Load” situations. For details, see High Load Problems